This application claims the priority benefit of Taiwan application Ser. No. 89101794, filed Feb. 2, 2000.
1. Field of the Invention
The invention relates in general to a light emitting diode. More particularly, this invention relates to a high efficiency white light emitting diode.
2. Description of the Related Art
The light emitting diode is a semiconductor device with high conversion efficiency to convert an electric energy into an optical energy. The theory of the light source is a junction of a P-type semiconductor and an n-type semiconductor. When the P-type and N-type semiconductors are connected to a positive and a negative electrodes, respectively, a forward bias is applied, the holes in the P-type and the electrons in the N-type semiconductor combine at the PN junction to emit a light. Currently, various light emitting diodes such as red light emitting diodes, yellow light emitting diodes have been developed and mass-produced.
The white light emitting diode using a blue light emitting diode as a light source has been developed. Using a fluorescent powder to partly absorb the blue light and to convert it into yellow or courge green (yellowish green) light, the remaining blue light and the converted yellow or courge green light are combined as a white light. FIG. 1 shows a structure of a conventional white light emitting diode disclosed by Nichia Chemical Industries, LTD in European Patent EP0936682. As shown in FIG. 1, the structure using a blue light emitting diode 102. The emitting surface is upwardly disposed, while the substrate 104 is downwardly disposed in a reflector 106 of a lead frame 112. Using wire bonding, the conductive wires 108 are fabricated. A fluorescent power of yttrium aluminum garnet (YAG) containing ccrium (Ce) is used to cover the blue light emitting diode 102. A molding step is then performed using an epoxy 110. A white light emitting diode 100 is fabricated in a form of a lamp.
The above white light emitting diode has a smooth surface. According to Snell Law, the critical angle xcex8c=sinxe2x88x921(n2/n1), wherein n1 and n2 are the refractive index of the semiconductor and the surrounding material. In this example, n1=3.4, n2=1.5, and xcex8c=27xc2x0, so that only the light at a cone with an angle of 2xcex8c can project through the semiconductor surface. Therefore, the efficiency of the output light is seriously restricted. In addition, using the above arrangement, even the substrate is facing downwardly, the P-electrode and the N-electrode co-exist in the light surface, so that a part of the electrode surface blocks the projection of light generated by the chip. The light efficiency is further reduced.
FIG. 2 shows a conventional flip chip (F/C) type blue emitting diode and the fabrication method thereof. The disclosure is polished in World Patent WO 98/34285 by Matsushita Electronics Corporation. In FIG. 2, the blue light emitting diode 200 has its transparent substrate 212 disposed upwardly, and the light emitting diode chip facing downwardly. The P-electrode 204a and N-electrode 204b are connected to the N-electrode 206a and P-electrode 206b respectively via the gold micro bumps 214a and 214b. The silicon substrate 202 is mounted on the lead frame 208. The conductive wires 210 are formed by wire bonding. Further by performing a molding step, the blue light emitting diode is formed.
In the above blue light emitting diode, the blue light emitting diode chip is mounted on the silicon substrate. When an operation current is 20 milli-Ampere (mA), the thermal and the electrical properties of the silicon substrate can be used as the submount of the light emitting diode chip. However, for a high efficiency light emitting diode, the operation current is larger, so that the demand on the properties of the submount is higher. The silicon substrate is thus not suitable for being used as the submount any more.
The invention provides a white light emitting diode. The surface of the light emitting diode chip is roughened to increase the probability of light entering the critical angle, so as to increase the output efficiency. In addition, using a flip chip structure, the blockage of light by the electrode is avoided to further enhance the light efficiency. The invention adapts a material with a high thermal conductivity as the submount, both the output and input efficiency are enhanced.
The high efficiency white light emitting diode provided by the invention comprises a light emitting diode chip, a transparent substrate, a transparent Ohmic electrode, a reflection layer, a contact electrode and a submount with a conductive trace. The transparent substrate having a roughened surface is disposed on a first surface of the light emitting diode chip. The transparent Ohmic electrode is located on a second surface of the light emitting diode chip. The reflection layer is formed on the transparent Ohmic electrode. The contact electrode is disposed on the reflection layer and connected to the Ohmic electrode. A submount with a high thermal conductivity is used to carry the light emitting diode chip. The conductive trace on the submount is electrically connected to the contact electrode on the chip by soldering material. The chip surface further comprises a fluorescent paste to absorb a part of the light emitted by the light emitting diode and to emit a complementary light to enable an observer to see a white light.
Both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.